The present invention relates to a charged particle beam exposure system, and especially to a method and apparatus of compensating for distortion caused by an eddy current induced by leakage flux of a magnetic lens.
Electron beam exposure and ion beam implantation apparatus are widely used in modern semiconductor manufacturing. As the integration scale becomes large in an integrated circuit (IC), requirements for fine patterning on a large area increase. Since it is difficult to maintain a sharply focused beam over a wide scanning area, the pattern to be exposed on a chip is divided into several fields. After one field is exposed, the stage is slid to the next field to expose the next field. In this manner, each of the fields are exposed one by one to complete the exposure over the entire pattern of the chip. The size of the field which can be scanned by the beam becomes smaller when the accuracy requirements of the pattern increase.
Recent ICs require an extremely fine pattern, so that the size of the field has become smaller to obtain the maximum sharp focusing over the entire field. On the other hand, the chip size to be exposed has become larger. For example, a pattern of a large scale integrated circuit (LSI) of 1.times.1 cm.sup.2 size is divided into fields of 1.times.1 mm.sup.2 size. Therefore, one hundred separate exposure are necessary for exposing one chip. This increases the time required to expose the chip and decreases the throughput of the beam exposure systems. Further, the tendency in recent LSI manufacturing is for further increases in exposure time per chip.
In order to overcome the above problem, a continuously moving stage system has been deemed necessary. This system can expose the beam over the field while the stage carrying the substrate is moving. However, a new problem has resulted from this continuous exposure system. This new problem, the eddy current problem, was not found in prior systems such as the step and repeat system, because in the step and repeat system the stage is fixed while the exposure is carried out, and the beam is cut off while the stage is moving. No eddy current occurs while the beam is exposing the substrate. Further details of the step and repeat system can be found, for example, in U.S. application Ser. No. 689,010, filed on Jan. 4, 1985, by H. Yasuda et al.
There are systems in which the stage is moved while the electron beam is irradiating the substrate to expose a large pattern. In such continuous exposure systems, the problem of eddy current is not so serious, because in such systems, the bore of the magnetic lens through which the electron beam is projected is designed to be small to reduce the leakage flux as much as possible. Further, the focal length of the magnetic lens can be chosen longer than that of the step and repeat system, so that a working distance, i.e., a gap between the lens and the stage, is large. Accordingly, the leakage flux at the position of the stage is not so strong. However, since the scanning field of the continuously moving system is very narrow, the throughput is not so good. Further details of the continuously exposing system can be found for example in U.S. Pat. No. 4,362,942 by H. Yasuda, or "A HIGH DOSE AND HIGH ACCURACY VARIABLE SHAPED ELECTRON BEAM EXPOSURE SYSTEM FOR QUATERMICRON DEVICE FABRICATION" by R. Yoshikawa et al., J. Vac. Technol. B5 (1), Jan./Feb. 1987, American Vacuum Society.
If the stage in the step and repeat system is moved while the beam is exposing the substrate, the problem of eddy current occurs, because the bore of the magnetic lens is made large to obtain a wide scanning field, and the working distance is short compared to that of the continuously moving exposure system in order to maintain the high resolution. Therefore, the leakage flux from the magnetic lens cannot be neglected, and a deviation in the path of the electron beam is caused by the eddy current.